1/7/2013. Overview: Locating Genes Along Chromosomes. Morgan s Experimental Evidence: Scientific Inquiry. Morgan s Choice of Experimental Organism

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1 / 4 / 4 / 4 / 4 R y yr /7/203 Ovevie: Locating Genes Along Chomosomes Mendel s heeditay factos ee genes Today e can sho that genes ae located on chomosomes The location of a paticula gene can be seen by tagging isolated chomosomes ith a fluoescent dye that highlights the gene Concept 5.: Mendelian inheitance has its physical basis in the behavio of chomosomes Mitosis and meiosis ee fist descibed in the late 800s The chomosome theoy of inheitance states: Mendelian genes have specific loci (positions) on chomosomes Chomosomes undego segegation and independent assotment The behavio of chomosomes duing meiosis can account fo Mendel s las of segegation and independent assotment Figue 5.2 P ello-ound Geen-inkled seeds (RR) seeds (yy) y R R y Meiosis Fetilization R y Gametes All F plants poduce yello-ound seeds (yr). F R R y y Meiosis LAW OF SEGREGATION LAW OF INDEPENDENT The to alleles fo each R R ASSORTMENT Alleles of genes gene sepaate duing on nonhomologous chomosomes gamete fomation. Metaphase I assot independently duing gamete fomation. y y R R Anaphase I y y R R 2 2 y Metaphase II y Mogan s Expeimental Evidence: Scientific Inquiy The fist solid evidence associating a specific gene ith a specific chomosome came fom Thomas Hunt Mogan, an embyologist Mogan s expeiments ith fuit flies povided convincing evidence that chomosomes ae the location of Mendel s heitable factos y y y y Gametes R R R R F 2 An F F coss-fetilization 3 Fetilization ecombines 3 the R and alleles at andom. 9 : 3 : 3 : Fetilization esults in the 9:3:3: phenotypic atio in the F 2 geneation. Mogan s Choice of Expeimental Oganism Seveal chaacteistics make fuit flies a convenient oganism fo genetic studies They poduce many offsping A geneation can be bed evey to eeks They have only fou pais of chomosomes Mogan noted ild type, o nomal, phenotypes that ee common in the fly populations Taits altenative to the ild type ae called mutant phenotypes

2 Coelating Behavio of a Gene s Alleles ith Behavio of a Chomosome Pai Figue 5.4 EPERIMENT P F RESULTS All offsping had ed eyes. In one expeiment, Mogan mated male flies ith hite eyes (mutant) ith female flies ith ed eyes (ild type) The F geneation all had ed eyes The F 2 geneation shoed the 3: ed:hite eye atio, but only males had hite eyes Mogan detemined that the hite-eyed mutant allele must be located on the chomosome Mogan s finding suppoted the chomosome theoy of inheitance F 2 CONCLUSION P Eggs F F 2 Eggs + + Concept 5.2: Sex-linked genes exhibit unique pattens of inheitance In humans and some othe animals, thee is a chomosomal basis of sex detemination The Chomosomal Basis of Sex In humans and othe mammals, thee ae to vaieties of sex chomosomes: a lage chomosome and a smalle chomosome Only the ends of the chomosome have egions that ae homologous ith coesponding egions of the chomosome The SR gene on the chomosome codes fo a potein that diects the development of male anatomical featues Figue Paents 44 + Females ae, and males ae Each ovum contains an chomosome, hile a spem may contain eithe an o a chomosome Othe animals have diffeent methods of sex detemination o Egg o (a) The - system Zygotes (offsping) (b) The -0 system ZW ZZ (c) The Z-W system 32 (Diploid) 6 (Haploid) (d) The haplo-diploid system 2

3 Inheitance of -Linked Genes A gene that is located on eithe sex chomosome is called a sex-linked gene Genes on the chomosome ae called -linked genes; thee ae fe of these Genes on the chomosome ae called -linked genes chomosomes have genes fo many chaactes unelated to sex, heeas the chomosome mainly encodes genes elated to sex detemination Figue 5.7 -linked genes follo specific pattens of inheitance Fo a ecessive -linked tait to be expessed A female needs to copies of the allele (homozygous) A male needs only one copy of the allele (hemizygous) -linked ecessive disodes ae much moe common in males than in females N N n N n N N n n n N n Eggs N N n N Eggs N N N N Eggs N N n N N N n N n N n n n n n n (a) (b) (c) Inactivation in Female Mammals Some disodes caused by ecessive alleles on the chomosome in humans Colo blindness (mostly -linked) Duchenne muscula dystophy Hemophilia In mammalian females, one of the to chomosomes in each cell is andomly inactivated duing embyonic development The inactive condenses into a Ba body If a female is heteozygous fo a paticula gene located on the chomosome, she ill be a mosaic fo that chaacte 3

4 Figue 5.8 Ealy embyo: To cell populations in adult cat: Active Black fu chomosomes Allele fo oange fu Cell division and chomosome inactivation Inactive Allele fo black fu Oange fu Active Concept 5.3: Linked genes tend to be inheited togethe because they ae located nea each othe on the same chomosome Each chomosome has hundeds o thousands of genes (except the chomosome) Genes located on the same chomosome that tend to be inheited togethe ae called linked genes Ho Linkage Affects Inheitance Figue EPERIMENT P (homozygous) Wild type (gay body, nomal ings) Double mutant (black body, vestigial ings) Mogan did othe expeiments ith fuit flies to see ho linkage affects inheitance of to chaactes Mogan cossed flies that diffeed in taits of body colo and ing size b + vg + F dihybid (ild type) b + + vg Testcoss offsping TESTCROSS Eggs b + vg + Wild type Blackvestigiavestigial Gay- Blacknomal (gay-nomal) b vg Double mutant b vg b + bvg + vg b vg b + bvg vg b + vg PREDICTED RATIOS If genes ae located on diffeent chomosomes: : : : If genes ae located on the same chomosome and paental alleles ae alays inheited togethe: : : 0 : 0 RESULTS 965 : 944 : 206 : 85 Figue 5.UN0 Mogan found that body colo and ing size ae usually inheited togethe in specific combinations (paental phenotypes) He noted that these genes do not assot independently, and easoned that they ee on the same chomosome F dihybid female and homozygous ecessive male in testcoss Most offsping o 4

5 Genetic Recombination and Linkage Hoeve, nonpaental phenotypes ee also poduced Undestanding this esult involves exploing genetic ecombination, the poduction of offsping ith combinations of taits diffeing fom eithe paent The genetic findings of Mendel and Mogan elate to the chomosomal basis of ecombination Figue 5.UN02 Recombination of Unlinked Genes: Independent Assotment of Chomosomes Mendel obseved that combinations of taits in some offsping diffe fom eithe paent Offsping ith a phenotype matching one of the paental phenotypes ae called paental types Offsping ith nonpaental phenotypes (ne combinations of taits) ae called ecombinant types, o ecombinants A 50% fequency of ecombination is obseved fo any to genes on diffeent chomosomes Gametes fom geeninkled homozygous ecessive paent (yy) y Gametes fom yello-ound dihybid paent (yr) R y yr yr yy y yyr Paentaltype offsping Recombinant offsping Recombination of Linked Genes: Cossing Ove Mogan discoveed that genes can be linked, but the linkage as incomplete, because some ecombinant phenotypes ee obseved He poposed that some pocess must occasionally beak the physical connection beteen genes on the same chomosome That mechanism as the cossing ove of homologous chomosomes Figue 5.0 Testcoss paents Testcoss offsping Eggs Gay body, nomal ings (F dihybid) Replication of chomosomes Meiosis I b + vg + Meiosis II Recombinant chomosomes 965 Wild type (gay-nomal) b + vg + b + vg 944 Blackvestigial 206 Gayvestigial 85 Blacknomal Black body, vestigial ings (double mutant) Replication of chomosomes + Meiosis I and II Paental-type offsping Recombinant offsping Recombination 39 ecombinants = 00 = 7% fequency 2,300 total offsping 5

6 Ne Combinations of Alleles: Vaiation fo Nomal Selection Recombinant chomosomes bing alleles togethe in ne combinations in gametes Random fetilization inceases even futhe the numbe of vaiant combinations that can be poduced This abundance of genetic vaiation is the a mateial upon hich natual selection oks Mapping the Distance Beteen Genes Using Recombination Data: Scientific Inquiy Alfed Stutevant, one of Mogan s students, constucted a genetic map, an odeed list of the genetic loci along a paticula chomosome Stutevant pedicted that the fathe apat to genes ae, the highe the pobability that a cossove ill occu beteen them and theefoe the highe the ecombination fequency A linkage map is a genetic map of a chomosome based on ecombination fequencies Distances beteen genes can be expessed as map units; one map unit, o centimogan, epesents a % ecombination fequency Map units indicate elative distance and ode, not pecise locations of genes Genes that ae fa apat on the same chomosome can have a ecombination fequency nea 50% Such genes ae physically linked, but genetically unlinked, and behave as if found on diffeent chomosomes Stutevant used ecombination fequencies to make linkage maps of fuit fly genes Using methods like chomosomal banding, geneticists can develop cytogenetic maps of chomosomes Cytogenetic maps indicate the positions of genes ith espect to chomosomal featues Concept 5.4: Alteations of chomosome numbe o stuctue cause some genetic disodes Lage-scale chomosomal alteations in humans and othe mammals often lead to spontaneous abotions (miscaiages) o cause a vaiety of developmental disodes Plants toleate such genetic changes bette than animals do 6

7 Abnomal Chomosome Numbe In nondisjunction, pais of homologous chomosomes do not sepaate nomally duing meiosis As a esult, one gamete eceives to of the same type of chomosome, and anothe gamete eceives no copy Figue Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n + n + n n n + n n n (a) Nondisjunction of homologous chomosomes in meiosis I Numbe of chomosomes (b) Nondisjunction of siste chomatids in meiosis II Aneuploidy esults fom the fetilization of gametes in hich nondisjunction occued Offsping ith this condition have an abnomal numbe of a paticula chomosome A monosomic zygote has only one copy of a paticula chomosome A tisomic zygote has thee copies of a paticula chomosome Alteations of Chomosome Stuctue Polyploidy is a condition in hich an oganism has moe than to complete sets of chomosomes Tiploidy (3n) is thee sets of chomosomes Tetaploidy (4n) is fou sets of chomosomes Polyploidy is common in plants, but not animals Polyploids ae moe nomal in appeaance than aneuploids Beakage of a chomosome can lead to fou types of changes in chomosome stuctue Deletion emoves a chomosomal segment Duplication epeats a segment Invesion eveses oientation of a segment ithin a chomosome Tanslocation moves a segment fom one chomosome to anothe 7

8 Figue 5.4 (a) Deletion A B C D E F G H A B C E F G H (b) Duplication (c) Invesion (d) Tanslocation A deletion emoves a chomosomal segment. A B C D E F G H A duplication epeats a segment. A B C B C D E F G H A B C D E F G H An invesion eveses a segment ithin a chomosome. A D C B E F G H A B C D E F G H M N O P Q R A tanslocation moves a segment fom one chomosome to a nonhomologous chomosome. E F G M N O C H D A B P Q R Human Disodes Due to Chomosomal Alteations Alteations of chomosome numbe and stuctue ae associated ith some seious disodes Some types of aneuploidy appea to upset the genetic balance less than othes, esulting in individuals suviving to bith and beyond These suviving individuals have a set of symptoms, o syndome, chaacteistic of the type of aneuploidy Figue 5.5b Don Syndome (Tisomy 2) Don syndome is an aneuploid condition that esults fom thee copies of chomosome 2 It affects about one out of evey 700 childen bon in the United States The fequency of Don syndome inceases ith the age of the mothe, a coelation that has not been explained Aneuploidy of Sex Chomosomes Nondisjunction of sex chomosomes poduces a vaiety of aneuploid conditions Klinefelte syndome is the esult of an exta chomosome in a male, poducing individuals Monosomy, called Tune syndome, poduces 0 females, ho ae steile; it is the only knon viable monosomy in humans Disodes Caused by Stuctually Alteed Chomosomes The syndome ci du chat ( cy of the cat ), esults fom a specific deletion in chomosome 5 A child bon ith this syndome is mentally etaded and has a catlike cy; individuals usually die in infancy o ealy childhood Cetain cances, including chonic myelogenous leukemia (CML), ae caused by tanslocations of chomosomes 8

9 Concept 5.5: Some inheitance pattens ae exceptions to standad Mendelian inheitance Thee ae to nomal exceptions to Mendelian genetics One exception involves genes located in the nucleus, and the othe exception involves genes located outside the nucleus In both cases, the sex of the paent contibuting an allele is a facto in the patten of inheitance Genomic Impinting Fo a fe mammalian taits, the phenotype depends on hich paent passed along the alleles fo those taits Such vaiation in phenotype is called genomic impinting Genomic impinting involves the silencing of cetain genes that ae stamped ith an impint duing gamete poduction Figue 5.7 Patenal chomosome Matenal chomosome (a) Homozygote Nomal Igf2 allele is expessed. Nomal Igf2 allele is not expessed. Mutant Igf2 allele inheited fom mothe Nomal-sized mouse (ild type) Nomal Igf2 allele is expessed. Nomal-sized mouse (ild type) Mutant Igf2 allele inheited fom fathe Daf mouse (mutant) Mutant Igf2 allele is expessed. It appeas that impinting is the esult of the methylation (addition of CH 3 ) of cysteine nucleotides Genomic impinting is thought to affect only a small faction of mammalian genes Most impinted genes ae citical fo embyonic development Mutant Igf2 allele is not expessed. (b) Heteozygotes Nomal Igf2 allele is not expessed. Inheitance of Oganelle Genes Extanuclea genes (o cytoplasmic genes) ae found in oganelles in the cytoplasm Mitochondia, chlooplasts, and othe plant plastids cay small cicula DNA molecules Extanuclea genes ae inheited matenally because the zygote s cytoplasm comes fom the egg The fist evidence of extanuclea genes came fom studies on the inheitance of yello o hite patches on leaves of an otheise geen plant Some defects in mitochondial genes pevent cells fom making enough ATP and esult in diseases that affect the muscula and nevous systems Fo example, mitochondial myopathy and Lebe s heeditay optic neuopathy 9